The subject matter of the present invention relates to a new method and apparatus for processing a signal which is output from a pulsed nuclear magnetism tool disposed in a wellbore thereby producing new output data representative of the formation traversed by the wellbore and for recording the new output data on an output record medium.
Repeated attempts have been made to use the principles of nuclear magnetic resonance by means of logging tools lowered through wellbores in oil exploration over the past several decades. It is recognized that particles of a formation having magnetic spin, for example atomic nuclei, protons, or electrons, have tendencies to align with a magnetic field which is imposed on the formation. Such a magnetic field may be naturally generated, as is the case with the earth's magnetic field (B.sub.E) which has an intensity of approximately 0.5 gauss in areas of the globe where boreholes are typically drilled. Any given particle in a formation is additionally influenced by localized magnetic fields associated with nearby magnetic particles, other paramagnetic materials, and the layer of ions which typically line pore walls of certain types of formations such as shales. These localized fields tend to be inhomogeneous, while the earth's magnetic field is relatively homogeneous.
A nuclear magnetic resonance (NMR) logging tool apparatus, adapted to be disposed in a borehole, produces a static and a substantially homogeneous magnetic field focussed into a formation on one side of the logging tool. By directing and configuring the combined magnetic fields of a plurality of magnets, a region, remote from the plurality of magnets, is introduced wherein the special field gradient substantially vanishes, thereby insuring that the field is highly homogeneous throughout that region. In a preferred form, the magnets are mounted within a metallic skid or logging pad, the static magnetic field is directed through the face of the pad into an adjacent formation, and the region of substantially homogeneous field is situated in a volume of formation behind the mudcake layer which typically lines borehole walls. A homogeneous magnetic field, several hundred times stronger than the earth's magnetic field, can be thus imposed, or "focused", on a volume of formation in situ.
Reference may be had to U.S. Pat. No. 4,933,638 issued Jun. 12, 1990 to Kenyon et al (hereinafter termed, the "Kenyon et al patent" or "Kenyon et al") for details of such a nuclear magnetic resonance (NMR) logging tool apparatus, which patent is incorporated herein by reference. In the Kenyon et al patent, an RF antenna is mounted on the outside of the structure of the pad so that the pad serves as a natural shield against any signals which may be generated by resonant conditions behind the pad, particularly those potentially strong resonance signals from borehole fluid. In the preferred form, the antenna is configured to focus its signals radially outwardly from the pad face, into the volume of formation having the homogeneous field, thereby additionally reducing the distortion of measured signals from borehole effects.
All such nuclear magnetic resonance logging tool apparatus, when disposed in a borehole, are electrically connected to a computing apparatus disposed at the surface of the borehole. The computing apparatus stores a signal processing software therein, the software in conjunction with the hardware of the computing apparatus producing a plurality output data representative of the characteristics of the formation traversed by the borehole when the software is executed by the hardware while utilizing a set of input data which was developed by the logging tool disposed in the borehole.
While the prior art nuclear magnetic resonance logging tool of Kenyon et al, and its associated signal processing software, is capable of determining formation characteristics with sufficient accuracy and dependability, it has been found useful to improve the performance and accuracy of such logging tool, especially in view of the inherent difficulties of making nuclear magnetic resonance (NMR) measurements in boreholes.
One very important improvement in the performance of the Kenyon et al logging tool can be made to the signal processing software used by Kenyon et al. Several approaches to spectral decomposition or signal processing of NMR data have been reported. Spectral decomposition is a signal processing method that determines from NMR spin-echo signals in rocks, the individual amplitude components of the multi-exponential signal. These individual components correspond to different pore sizes in the rock.
A first approach is reported by Kenyon, W. E., Howard, J. J., Sezginer, A. Straley, C., Matteson, A., Horkowitz, R., and Erlich, R., in an article entitled "Poresize Distribution and NMR in Microporous Cherty Sandstones", Trans of the SPWLA of the 30th Ann. Logging Symp., Paper LL, Jun. 11-14, 1989, this first approach being further set forth in an article entitled "A NMR Technique for the Analysis of Pore Structure: Determination of Continuous Pore Size Distributions"; Gallegos, D. P. and Smith, D. M.; J. of Colloid and Interface Science, V. 122, No. 1, pp. 143-153, Mar. 1988, the disclosures of which are incorporated by reference into this specification.
A second approach is set forth in an article entitled "Problems in Identifying Multimodal Distributions of Relaxation Times for NMR in Porous Media", Magnetic Resonance Imaging, Vol 9, pp. 687-693 (1991), the disclosure of which is incorporated by reference into this specification.
The aforementioned approaches are computationally too intensive to be done in real time by a logging truck computer. They also do not compress the data which is needed to limit the telemetry requirements for sending data uphole. In addition, the NMR logging tool should first acquire downhole spin-echo measurements in earth formations penetrated by a borehole and then, secondly, generate a set of detailed formation evaluation information. However, this type of detailed formation evaluation information was previously obtainable only from costly laboratory analysis of conventional core data. Therefore, a signal processing method and apparatus is needed which is adapted to extract this formation evaluation information from the measured spin-echos. This signal processing method and apparatus must be capable of providing a spectral decomposition of the measured data, and it must be efficient and robust for real time processing of measured data during the acquisition of the data by an NMR logging tool moving in the borehole. Moreover, it is desirable to compress the data by elimination of redundant information. This reduces the telemetry requirements of the NMR tool, which is important if the tool is run in combination with other logging tools.